The critical back end step for HBM is thermal compression bonding - where they actually compress 13 very thin die separated by solder balls. Cracking is a major issue.
We are getting so many technologies like this that are so insanely specialized and require such deep knowledge of physical processes that some event like a 1918 flu that kills 10% of the population could take out enough people that we'd be unable to make things like this for years as we fill in the missing knowledge workers.
It's just crazy how far down the scale we're tickling atoms these days.
Surely we'd have had the same issue during the Spanish flu? Any examples?
Not really at all. Between those dates and now the world has become fully global and concentration of knowledge has massively occurred. The past world was much more highly distributed, far less technical, and far more redundant.
Maybe Covid killed a smaller but nonzero amount of technologies?